Viscosity of a fluid is a measure of its resistance to flow. Namely, viscosity means internal friction of a fluid in motion. Mathematically, viscosity is expressed as the ratio of tangential friction per unit area to velocity gradient perpendicular to flow direction of a fluid.
A viscometer is an instrument that measures viscosity of a fluid. Currently, well-used viscometers are capillary viscometer, rotational viscometer, etc. Measuring principle and function of such viscometers are as follows.
The rotational viscometer is an instrument that measures viscosity of a fluid by measuring the resistance caused by fluid in motion to a cylinder or a disk. The rotational viscometer, though appropriate for measuring viscosity within intermediate shear rate range, is not appropriate for measuring viscosity within zero shear rate range.
The capillary viscometer is an instrument that measures viscosity of a fluid by measuring mass flow and falling pressure of a fluid in a steady flow state and then using Poiseuille's Law. However, in case of using capillary viscometer to measure the viscosity, capillary ought to be precisely calibrated because viscosity is proportional to biquadrate of capillary diameter.
More particularly, in case of using disposable capillary viscometer to measure the viscosity of blood, it is difficult to calibrate every disposable fluid tube accurately. Furthermore, capillary ought to be perfectly cleansed after the calibration. If a capillary is not calibrated, in fact, accuracy of the measured value of the blood viscosity cannot be guaranteed.
The mechanical methods for measuring viscosity by the above arts are difficult to be applied particularly in diagnosis instruments or examination instruments, due to excessive amount of fluid consumed and pollution.
In case of image-based viscosity measuring method, the method requires little amount of fluid, costs little, and is able of quick measurement, but is difficult to measure with accuracy. The reason is that, in case of measuring the viscosity using the natural frequency of a droplet, the natural frequency of the droplet is hardly affected by viscosity. Furthermore, in case of measuring the viscosity by using an amplitude of a droplet, it is difficult to measure with accuracy because the amplitude of the droplet is subtly affected by not only viscosity but also volume, surface tension and density of the droplet and amplitude of the droplet vibrator, and these diverse variables cannot be accurately calibrated.
The present inventor completed the present invention having found that the ratio of the dynamic curvature change rate of a vibrating droplet to the static curvature change rate of the droplet is only affected by the viscosity of a fluid.